D

UNIT 1

BASIC ELECTRONICS DBV20023 UNIT 1: SEMICONDUCTOR DIODES

LEARNING OBJECTIVES

  • Discuss the basic structure of atoms.

  • Discuss semiconductors, conductors, and insulators and their differences.

  • Discuss covalent bonding in silicon.

  • Describe how current is produced in a semiconductor.

  • Describe the properties of n-type and p-type semiconductors.

  • Describe a diode and the formation of a pn junction.

  • Discuss diode biasing.

  • Analyze the voltage-current (V-I) characteristic curve of a diode.

  • Discuss diode operation.

LEARNING OUTCOMES

Upon completion, students should be able to:

  • Describe atomic makeup and the relationship between valence electrons and conductivity.

  • List principles governing associations between electrons and orbital shells.

  • Contrast trivalent vs pentavalent elements.

  • Highlight similarities and differences between n-type and p-type semiconductors.

  • Explain depletion layer formation around a pn junction.

  • Describe methods for forward and reverse biasing a pn junction.

1.1 ATOMIC STRUCTURE

  • Atoms consist of:

    • Electrons (negative charge)

    • Protons (positive charge)

    • Neutrons (neutral)

  • Nucleus contains protons and neutrons.

1.2 ELECTRON SHELLS AND ORBITS

  • Orbits grouped into energy bands (shells).

  • Each shell has a fixed max number of electrons.

  • Energy level increases as distance from the nucleus increases.

1.2.1 Energy Level

  • Defined energy states for electrons.

1.2.2 Valence Electron

  • The outermost electron in an atom.

  • Higher energy electrons are less tightly bound.

1.2.3 Ionization

  • Absorption of energy raises electrons' energy level.

  • Valence electron escapes, resulting in a positive ion and a free electron.

1.2.4 Max Electrons in Shell

  • Max electrons (Ne) per shell = 2n²

  • Example: Shell 1 can hold 2 electrons.

1.3 ELECTRICAL MATERIAL PROPERTIES

Conductors

  • Do not conduct electricity well; valence electrons are tightly bound.

Semiconductors

  • Intrinsic state neither a good conductor nor insulator (e.g., silicon, germanium).

  • 4 valence electrons.

Insulators

  • Easily conduct electrical current (e.g., copper, aluminum).

  • 1 loosely bound valence electron.

1.3.4 Energy Bands

  • Energy gap between valence and conduction bands signifies the energy needed for electrons to transition.

1.4 SEMICONDUCTOR ATOM COMPARISON

  • Silicon is the predominant material in diodes, transistors, and integrated circuits.

1.5 COVALENT BOND

  • Covalent bonds occur through the sharing of electrons with neighboring atoms.

1.6 CONDUCTION IN SEMICONDUCTORS

Energy Diagram

  • Intrinsic silicon has unexcited atoms with no electrons in the conduction band.

1.6.1 Conduction Electrons and Holes

  • Free electrons in the conduction band create holes in the valence band.

  • These contribute to electrical conductivity.

1.6.2 Electron and Hole Current

  • Voltage applies creates free electrons, which move toward the positive, generating current.

1.7 DOPING PROCESS IN SEMICONDUCTORS

  • Impurities added to increase free electrons or holes, thereby enhancing conductivity.

1.7.1 N-Type Semiconductor

  • Pentavalent atoms (e.g., Arsenic, Phosphorus) increase electron count in silicon.

1.7.2 P-Type Semiconductor

  • Trivalent atoms (e.g., Boron, Indium) increase hole count in silicon.

1.8 THE PN JUNCTION

  • Formed at the interface of P-type and N-type materials, creating a diode.

1.8.1 Depletion Region

  • Acts as a barrier to prevent electron movement across the junction.

1.8.2 Barrier Potential

  • Energy difference creates a potential barrier (e.g., 0.3V for germanium, 0.7V for silicon).

1.9 BIASING THE PN JUNCTION

1.9.1 Forward Bias

  • Current flows when negative voltage pushes electrons through the junction.

1.9.2 Reverse Bias

  • Positive voltage pulls electrons away, widening the depletion region.

1.9.2.1 Reverse Current

  • Small current exists due to thermally generated minority carriers.

1.9.2.2 Reverse Breakdown

  • High voltages increase current significantly, leading to potential diode damage.

1.10 VOLTAGE AND CURRENT CHARACTERISTICS

1.10.1 Forward Bias

  • Current increases rapidly after reaching strip voltage of approximately 0.7V.

1.10.2 Reverse Bias

  • A small reverse current flows until breakdown voltage is reached, at which point current sharply increases.